The present invention relates to transparent or semi-transparent sheets which are configured to transmit certain light energy images having certain directions of propagation while reflecting, absorbing, or modifying images or energy from other directions. Such sheet material is useful for producing panels for controlled solar heating, in controlling light transmission in building windows, in greenhouses, as described in my prior patents but has further uses, for example, for computer display screens, and in imprinted packaging film. The relevant portions of the description of my previous applications or patents identified herein is incorporated by reference.
It has long been known to incorporate lenticular lenses in a surface of thin plastic sheet material and to place a printed image on the opposite surface of the sheet material arranged so that the apparent image as viewed from different directions depends upon the direction of viewing with three or more different images being presented to the viewer as the direction of viewing changes. Such sheet materials have been used to produce moving image novelties and to produce stereoscopic effects and three-dimensional illusions. It has also been known to produce partially transparent film material having opaque striations formed within the sheet to produce the effect of louvers within the transparent sheet material. Such louver simulating material has a high transmission coefficient for light energy in certain directions while having a very low or near-zero light transmission coefficient for other directions. The louver simulating sheet material could have louvers of light absorbing or light reflecting composition. Examples of such prior devices for directional transmission of light energy and images are shown in U.S. Pat. No. 3,653,138 (U.S. Cl. 40/130R) and U.S. Pat. No. 4,021,945 (U.S. Cl. 40/28C). Other patens disclosing lenticular lens arrays are cited as references in U.S. Pat. No. 5,303,525.
According to the present invention, a transparent or partially transparent sheet material is provided which has convex lenses formed on the front surface and concave lenses formed on the back surface of the sheet. In the preferred embodiments the convex lenses on the front surface will have a larger radius of curvature (and a longer focal length) than the concave lenses on the back surface. Also in the preferred embodiments the convex lenses with the larger radius of curvature will substantially cover the surface on which they are formed with substantially no intervening non-convex surfaces while the concave lenses with the smaller radius of curvature will also have a smaller aperture, and, as they are equal in number to the larger lenses, there will be a space between the small radius lenses which will normally be a non-concave and at least partially convex surface. This intervening non-concave surface may occupy approximately half the area of the sheet on the back side and may be imprinted by generally conventional printing techniques with a desired monochrome or multi-color image, for example, one suitable for packaging film or incorporation in a product package. As will later be explained, the preferred embodiment of the directional image transmission sheet is capable of presenting an effectively transparent characteristic with a high transmission coefficient and low distortion for viewing from one direction, such as perpendicular to the sheet. When viewed from another direction, such as at a side angle of 45xc2x0, the sheet does not appear to be transparent, but rather the printed matter on the back surface of the sheet is visible substantially without distortion.
There is an additional advantage in the at least partially convex surface. Between the small radius lenses is a convex accurate surface with substantially no flat or planar area. This avoids flat surfaces on the back surface with the concave lenses which could have a tendency to reflect light rays with harmful effects on the optical characteristics.
In addition to providing the features and advantages discussed above, it is an object of the present invention to provide a thin transparent plastic sheet material having convex lenses embossed or otherwise formed in a front surface thereof which match one-to-one with concave lenses embossed or otherwise formed in the back surface of the sheet with the radii of curvature of the lenses being determined to cause the focal point of each lens on the front surface to substantially coincide with the focal point of the corresponding lens on the back surface whereby a Galilean telescope effect is produced allowing images to be viewed through the sheet material substantially without distortion from a direction perpendicular or nearly perpendicular to the sheet material.
It is another object of the present invention to provide such a sheet material wherein the concave lenses on the back surface of the sheet have a radius of curvature of no greater than about three-quarters the radius of curvature of the convex lenses on the front surface of the sheet.
It is still another object of the present invention to provide such a sheet material wherein the lenses are elongated lenticular lenses arranged in a parallel fashion and wherein the width of the lenses is about equal to or less than about one millimeter (about 0.04 inches).
It is yet another object of the present invention to provide such a sheet material wherein the lenses on the back surface are concave and of substantially less width than those on the front surface so that there are intervening spaces between the concave lenses, and such surfaces are non-concave and preferably without planar portions while being adapted to be imprinted with a solid color or an information imparting pattern which may be viewed only from oblique angles from the front surface of the sheet material.